An agricultural implement is generally composed of a number of work units, such as seed or fertilizer dispensers, or soil preparation tools, e.g., discs, tillers, cultivators, plows, and the like, that are typically carried by an implement frame that is hitched to and towed by a tractor, combine or similar wheeled device. The implement frame is generally supported above the ground by a pair of frame supporting wheels, which are mounted to wheel mounting spars that are rigidly attached to a rockshaft. The rockshaft may be rotated by a hydraulic cylinder to effectively raise and lower the implement frame and thus the work units.
For many agricultural implements, it is necessary for an operator to manually raise and lower the implement frame and the hydraulic cylinder holds the implement frame at the position set by the operator. More particularly, conventional depth control systems utilize a poppet valve to stop the flow of hydraulic fluid to the hydraulic cylinder to set the depth of the implement frame and thus the work units. Such stop valves have been found to be inconsistent in setting the depth of the implement frame and the valve is set until hydraulic fluid flow is reversed.
More recently, implements have been designed whereby the depth of the implement frame is monitored and hydraulic fluid flow to the hydraulic cylinder is controlled accordingly. U.S. Pat. No. 6,076,611 to Rozendaal et al. discloses an implement mounted depth control system whereby an electronic position sensor is mounted to the implement frame and senses the rotational position of the rockshaft. The rotational position of the rockshaft is used by a monitor to derive a depth of the work units and control the hydraulic cylinder to raise or lower the implement frame to raise or lower the work units to an operator-selected depth. The depth control system further allows an operator to raise and lower the implement frame using controls within the operator cab.